A fluorinated polymer is excellent in e.g. heat resistance, chemical resistance, weather resistance and non-tackiness, but has difficulty in adhering to other materials so as to form a laminate or a composite material. Therefore, techniques have been developed, wherein a polar functional group is introduced to a fluorinated polymer so as to impart adhesion to other materials (e.g. Patent Documents 1 to 3).
In production of a fluorinated polymer having a polar functional group, a hydrochlorofluorocarbon such as 1,3-dichloro-1,1,2,2,3-pentafluoropropane, has been heretofore used as a chain transfer agent which also functions as a polymerization medium. Nevertheless, a hydrochlorofluorocarbon is an ozone layer-depleting substance, and therefore it has been desired to use a chlorine-free fluorinated polymerization medium.
However, since a polar functional group-containing monomer is usually less soluble in a chlorine-free fluorinated polymerization medium, it is difficult to convert a solid-form polar functional group-containing monomer into a solution form when supplied to a polymerization vessel. If the solid-form polar functional group-containing monomer is supplied to a polymerization vessel as it is, the polar functional group-containing monomer may be hardly taken into a fluorinated polymer due to dispersion failure in the polymerization vessel, and therefore the adhesion of a fluorinated polymer obtained tends to be low.
Further, in recent years, a polymerization medium to be used for producing a fluorinated polymer is often recycled, with a view to protecting environment and reducing the cost of raw materials.
However, as disclosed in Patent Document 3, if a polar functional group-containing monomer is dissolved in a chain transfer agent which is then continuously charged to a polymerization system, the concentration of the chain transfer agent becomes high in a mixed fluid containing a polymerization medium and the chain transfer agent, recovered after polymerization. Therefore, in the case of recycling the mixed fluid containing the polymerization medium and the chain transfer agent, recovered after polymerization, it is necessary to once separate the polymerization medium and the chain transfer agent, and then to readjust the chain transfer agent so as to be a predetermined amount or to newly add a polymerization medium so as to lower the concentration of the chain transfer agent, whereby production efficiency tends to deteriorate.